Transponder frequency proximity warning systems are represented in U.S. Pat. No. 3,713,161 by Rice. Transponder only systems detect the target object transponder replies to interrogations. These systems normally do not use relative bearing data.
Transponder only systems detect the bracket pulses of the target object replies. The range to the target object is computed by use of the signal strength of its reply. These systems suffer in accuracy from the wide range of transmitted power in airline and low cost general aviation transponders.
Also, the use of transponder reply bracket pulses without interrogation signal source pulse reference correlation filtering results in fruit and garble corruption of the data. This corruption produces a significant number of false alarms.
Without time reference to the interrogation pulses, the transponder signal processing systems cannot accurately determine if the transponder reply was in response to a mode A or a mode C interrogation. Therefore, transponder reply only systems cannot effectively separate target transponder reply altitude or identification data frames.
Previous systems obtain relative bearing data using traditional multiple receiver channel, monopulse amplitude or phase based antenna direction finding technology. These receiver designs require complicated and costly multichannel microwave receiver equipment.
U.S. Pat. No. 3,858,211 issued to Litchford uses predictive timing of synchronous interrogation signal source pulse reference to correlate the target object transponder reply pulses. This method produces accurate range and bearing of the transponder reply, using a single receiver rf channel omnidirectional antenna for processing synchronous interrogation signals. The Litchford differential timing pulse method improves on the simple transponder reply receive only bracket pulse designs. An interrogation signal source based timing reference is used to develop a differential time of arrival range with respect to the target object replies. This time of arrival data is used to calculate target object position. The Litchford correlated dual frequency signal processing method provides a means of filtering uncorrelated replies from desirable sources. The filtering is accomplished by providing a predicted synchronous timing reference for transponder replies to the interrogation pulses. Utilizing the predicted interrogation pattern as a reference provides the capability to extract identification and altitude data from the transponder reply signals. The Litchford time differential method relies on flywheel prediction of the time of arrival of the interrogation signal source pulse reaching the observer object when the interrogation signal source is pointing away from the observer object and no interrogation signal is being received.
The Litchford method works well for continuously synchronous interrogation signal source pulse repetition patterns. In the next few years, over fifty percent of the interrogation signal sources will be ASR-9. The synchronous Litchford design has difficulty with the ASR-9, because its interrogation pattern is not synchronous. The ASR-9 has a variable ratio short and long pulse repetition pattern. The number of the pulses is dependent on ASR-9 antenna scan rate variations. At the end of an approximate 1.4 degrees of antenna azimuth travel, the ASR-9 adds or deletes pulses, depending on how much the rotation rate of the antenna is modified by wind loading. The target object transponder replies will no longer be synchronized with a continuously predictable interrogation pulse pattern. Thus, target object transponder replies will become uncorrelated and random with respect to the synchronous predicted interrogation pulse reference of the Litchford design. This makes the differential time range prediction unusable, unless the interrogation signal source pulses can be continuously monitored and the actual interrogation pulse correlated with each target object transponder reply pulse. Reliable continuous receipt of the interrogation pulses can only be achieved near the interrogation signal source.
U.S. Pat. No. 4,910,526 issued to Donnangelo et al uses a multichannel phase monopulse antenna to obtain the relative bearing of the interrogation and the target object transponder replies. The system performs a three dimensional, direction cosine vector, Kalman filtered solution which requires external aircraft position data. For the case of multiple interrogation signal sources, the space position of the observer object is determined by relative bearing triangulation of the available interrogation signal sources. Observer object altitude data is also required.
The Donnangelo primary means of target position determination uses multiple interrogation signal source data, target object pointing angle, relative bearing target data along with observer and target altitude to compute the target position.
The Donnangelo solution degrades for multiple interrogation signal source triangulation solutions when target altitude data is not available and also for single interrogation signal source geometry solutions. In these cases, the Donnangelo system relies on the Litchford type flywheeled synchronous predictive delta time of arrival data. This delta time range data is used to iterate the solution to a target position and to correlate mode C altitude. This synchronous interrogation signal source reference data will not be available when using non synchronous ASR-9 type signal inputs.